The technique of hydroprocessing of hydrocarbons has been known and used for many years. By hydroprocessing is meant the reaction with hydrogen, usually in the presence of a catalyst, of a feedstock. Typical hydroprocessing processes include hydrodesulfurization, hydrodenitrification, hydrodemetalation, and hydrocracking; and two or more of these processes may be carried on in the same reactor, either in the same or different catalytic beds. Although hydroprocessing may be applied to any hydrocarbon feedstock, it is particularly applicable, though less easily applicable, to heavier feedstocks such as residua, vacuum and atmospheric gas oils, coal and shale liquids, etc., since these feedstocks typically contain higher concentrations of less easily removed contaminants.
It has been found that when hydrocarbon fractions having a wide boiling range, especially residua, etc., are hydroprocessed, the heavier and more polar fractions tend to dominate the hydroprocessing reaction, so that the lighter fractions are less completely reacted. This is believed to be because the heavier fractions are preferentially absorbed onto the catalyst surface and into the pores, leaving less catalytic surface area available for the lighter and less polar molecules. Thus, though the lighter molecules may be more readily hydroprocessed, e.g., to a lower sulfur content, if processed alone, they will not be as efficiently processed if heavier molecules are present. More extensive hydroprocessing, with a concomitant increase in hydrogen consumption, is thus necessary to achieve adequate treatment in the same reactor.
Various methods have been proposed to solve this problem. For example, an alternative for atmospheric residuum desulfurization is to vacuum distill the residuum and desulfurize the gas oil and vacuum residuum separately. See, e.g., U.S. Pat. Nos. 3,830,731 to Reed et al. and 3,902,991 to Christensen et al. These, however, require two reactors (and associated equipment) rather than one, and also require a vacuum distillation column. Wilson et al., U.S. Pat. No. 3,658,681, employ a single reactor with two feed streams, one downward through the upper beds and the other upward through the lower beds. Other multiple reactor techniques are shown in U.S. Pat. Nos. 3,617,525 to Moritz et al. and 4,173,528 to Frayer et al. Pappas et al., U.S. Pat. No. 3,091,586, show a multiple bed reactor with a single feed but multiple withdrawal points for intermediate effluents. Vernon et al., U.S. Pat. No. 4,434,045, show a hydrocracking technique where a light fraction is hydrotreated to provide a recycle donor solvent. The disclosures of these and other patents cited herein are incorporated herein by reference.
With the decreasing demand relative to predictions for petroleum products, many refineries are now operating below design capacity. Accordingly, it would be uneconomic in many instances to add additional hydroprocessing equipment, even though environmental regulations require steadily decreasing sulfur, etc., levels in fuels. A hydroprocessing technique that would improve overall efficiency without substantial capital or operations expenditure would thus be desirable.